at86rf231: replace vtimer_usleep by hwtimer_wait

This fixes a bug where the callback to unlock the thread, put to sleep by
`vtimer_usleep` wouldn't be woken up due to overhead. Using hwtimer_wait
works for now, but this section will be replaced a driver refactor anyway.

drivers/at86rf231/at86rf231.c

@@ -26,7 +26,7 @@
 #include "periph/spi.h"
 #include "kernel_types.h"
 #include "transceiver.h"
-#include "vtimer.h"
+#include "hwtimer.h"
 
 #define ENABLE_DEBUG (0)
 #include "debug.h"
@@ -101,7 +101,7 @@ void at86rf231_switch_to_rx(void)
     do {
         status = at86rf231_get_status();
 
-        vtimer_usleep(10);
+        hwtimer_wait(HWTIMER_TICKS(10));
 
         if (!--max_wait) {
             printf("at86rf231 : ERROR : could not enter RX_ON mode\n");
@@ -258,4 +258,3 @@ void at86rf231_reset(void)
     } while ((status & AT86RF231_TRX_STATUS_MASK__TRX_STATUS)
              != AT86RF231_TRX_STATUS__TRX_OFF);
 }
-